1. Field
Embodiments of the present invention relate to a polymer electrolyte and a lithium rechargeable battery including the same. More particularly, embodiments of the present invention relate to a polymer electrolyte having improved reliability and safety by increasing thermal stability of a polymer of the polymer electrolyte and crosslinking-density of a matrix of the polymer while improving electrode impregnation capability by inducing low viscosity in a pre-gel composition, and a lithium rechargeable battery including the same.
2. Description of the Related Art
Recently, due to reductions in the size and weight of portable electronic equipment, portable electronic equipment is increasingly being used. A battery having a high energy density for use as a power source of such portable electronic equipment is needed, and thus, research into a rechargeable lithium battery has been actively conducted. For a positive active material of a rechargeable lithium battery, a lithium-transition element oxide has been used. For a negative active material, a crystalline or amorphous carbon-based material or carbon composite has been used. To fabricate positive and negative electrodes, either the positive and negative active materials are coated on a current collector at an appropriate thickness, or the positive and negative active materials are made in the form of a film. To fabricate an electrode assembly, the positive and negative electrodes are then wound or stacked with an insulating separator interposed therebetween. The electrode assembly is put into a metal can or other battery container, such as a metal-laminated pouch, and an electrolyte solution is injected to fabricate a rechargeable battery.
Compared to using a can as a battery container, using a pouch as a battery container is advantageous in that the shape of the pouch may be flexibly changed and the size of the pouch may be increased. However, the pouch-type battery container is disadvantageous in that it may be easily deformed and damaged by external physical impact and it may be swollen when exposed to high temperatures. Such disadvantages are more serious for rechargeable lithium batteries using a liquid electrolyte solution than for rechargeable lithium batteries using a polymer electrolyte. Therefore, a pouch-type battery container is usually used with polymer electrolyte rechargeable lithium batteries.
The polymer electrolyte rechargeable lithium battery has advantages such as a reduced likelihood of leaking, improved safety, improved high-temperature stability, and it may be more resistant to external physical impact.
In a case of a chemical gel type polymer battery wherein the chemical gel type polymer electrolyte is formed by polymerization of a “pre-gel” after it has been injected inside a can (the “pre-gel” including a mixture of a monomer, an electrolyte solution and an initiator of battery), the viscosity of the “pre-gel” is greatly affected by the amount and molecular weight of the monomer of the “pre-gel.” It is advantageous for the electrolyte solution to have low viscosity so that the electrolyte solution can be uniformly impregnated into the electrode and the separator after being injected into the battery. In particular, intercalation-deintercalation of lithium ions may not occur at a separator portion, specifically an electrode portion, without an electrolyte solution impregnated therein during charging/discharging, resulting in a reduction in capacity. In addition, it is often the case that the reliability and safety of a battery may be affected by precipitated lithium, which may result if the electrolyte solution is not uniformly impregnated in the electrode and the separator.